In known hydraulic transmission systems, a prime mover drives a pump which converts power into hydraulic pressure. The hydraulic pressure is then transmitted to a hydraulic motor which converts the pressure back into power, which may then be used to power a load. In a closed loop circuit, the hydraulic fluid returns directly to the pump through a return line. In an open loop circuit, the hydraulic fluid returns to a reservoir, which feeds the pump. The pump may be a variable displacement pump and the motor may be a variable displacement motor.
In the context of prior art wind turbines, a rotor is the prime mover and the load is an electrical generator.